There has been known a roadside object detection apparatus which estimates the structure of the road surface around a vehicle using a parallax image captured by a stereo camera and detects a step on the road surface formed by roadside objects, such as a curb, from the structure of the road surface (see Japanese Patent Application Publication No. 2014-002608). The apparatus in Japanese Patent Application Publication No. 2014-002608 scans the height of the road surface along a horizontal scanning line of an image, and detects a step if the height change amount of the road surface is larger than or equal to a threshold.
Because of the influence of errors included in the structure of the road surface estimated from an image of the stereo camera, as the angle formed by the horizontal scanning line relative to the tangent direction of a step decreases, the gradient of the height change of the road surface becomes smaller. Accordingly, when the angle formed by the tangent of the step and the horizontal scanning line is small, the detection accuracy of the step position is low.